Crystallization of Lennard-Jones nanoparticles has been studied by molecular dynamics (MD) simulations. Spherical models with free surface are cooled from the melt to crystalline state. In the cooling process, thermodynamics, structural properties and atomic mechanism of the crystallization are investigated. We found that crystallization in nanoparticles follows the Ostwald’s step rule like that found in the past. Due to free surface contribution, the solidification exhibits non-homogeneous behavior which proceeds in different manners between core and surface: homogeneous crystallization in the core and heterogeneous one in the surface layer of nanoparticles. It is due to the discrepancy between structures of two parts: highly ordered structure dominates in the core region while the surface exhibits defective one with a high fraction of undercoordinated sites. Also, our results are consistent with previous ones about the free surface-induced phenomena.